This invention relates to fluid samplers, and more particularly to apparatus for performing non-destructive collection of fluid samples from subsurface earth formations traversed by a borehole.
The sampling of fluids contained in subsurface earth formations provide a method of testing formation zones of possible interest by recovering a sample of any formation fluids present for later analysis at the earth's surface while causing a minimum of damage to the tested formations. Thus, the formation sampler is essentially a point test of the possible producibility of subsurface formations. Additionally, a continuous record of the sequence of events during the test is made at the surface. From this record valuable formation pressure and permeability data can be obtained for formation reservoir analysis.
Early formation fluid sampling instruments were not fully successful as a commercial service because they were limited to a single test on each trip into the borehole. Later instruments were suitable for multiple testing, however, the success of these testers depended to some extent on the characteristics of the particular formations to be tested. For example, where earth formations were unconsolidated a different sampling apparatus was required than in the case of consolidated formations.
One of the major problems which has hampered the reliable testing of unconsolidated formations has been in designing a suitable system for establishing fluid communication with the formations. This problem is particularly accute in low pressure, low permeability, unconsolidated formations. For various reasons, systems designed for testing these formations have proven to be less than completely reliable. For example, U.S. Pat. No. 3,352,361, issued to Urbanosky, discloses of formation tester instrument including a tubular probe member having a tubular filter therein to capture formation particles entering the probe. U.S. Pat. No. 3,653,436, issued to Anderson et al, likewise has a filtering member, comprised of finely meshed screen within the tubular sampling probe. Usage has shown that in the aforementioned unconsolidated formation these filter members will become plugged, preventing formation fluids from passing therethrough and limiting reliable sampling to a single test.
U.S. Pat. No. 3,864,970, issued to Bell, attempts to overcome the problem of filter plugging by using two selectively sized filter passages, one size to pass plugging materials and the second sized to be smaller than the formation materials. Another technique can be found in U.S. Pat. No. 3,934,468, issued to Brieger, which utilizes a extendable filter probe which extends only when testing unconsolidated formations. Yet another technique is found in U.S. Pat. No. 4,248,081, issued to Hallmark, which uses a spiral spring within the tubular sampling probe as a filter member. The length of the spiral spring, and thus the filtering ability, is charged as the tubular sampling probe extends into the earth formations. While these advanced filter methods have provided some improvement in obtaining fluid samples in unconsolidated formations, they have not reliably prevented filter plugging where the unconsolidated formations are low pressure and low permeability.
Other efforts to increase reliability in obtaining samples of formation fluids in subsurface earth formations have encompassed various methods of cleaning the filter material. For example, U.S. Pat. No. 3,811,321, issued to Urbanosky, uses a selectively operable valve which is rapidly opened to place a low-pressure chamber and a flow line in the tool into communication with the isolated formation to remove plugging materials from a filtering median. U.S. Pat. No. 3,813,936, issued to Urbanosky et al, further attempts to clean the filter by taking in well fluids and discharging these fluids through the filter in reverse direction, into the earth formation for cleaning the filter before sampling. Yet another attempt to overcome the problem of filter plugging is illustrated in U.S. Pat. No. 3,952,588, issued to Whitten, which uses a selectively movable chamber to draw mud cake and other plugging materials into the chamber. Thereafter, the chamber is shifted to communicate with a screened entry port of the fluid admitting probe. While these advanced designs represent improvements, usage has shown that clayistic materials found in these problem earth formations are difficult to purge from a filtering member hampering the ability to consistantly obtain fluid samples in such formations.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing method and apparatus for obtaining a plurality of formation fluid samples under adverse formation conditions in a single traversal of the borehole.